1. Field of the Invention
The present invention relates to an optical system and an optical apparatus including the same, such as silver salt film cameras, digital still cameras, video cameras, telescopes, binoculars, projectors, and copying machines.
2. Description of the Related Art
It is desired that optical systems used in image pick-up apparatuses, such as digital cameras and video cameras, have a short optical overall length (length from the lens surface on the most object side to the image plane).
In general, as the size of an optical system is reduced, aberrations (particularly chromatic aberration, such as longitudinal chromatic aberration and lateral chromatic aberration) are liable to degrade the optical performance. In particular, in a telephoto optical system whose optical overall length has been reduced, as the focal length is increased, the occurrence of chromatic aberrations increases.
In order to reduce the occurrence of chromatic aberrations in an optical system, a material having extraordinary partial dispersion is often used as an optical material.
In a telephoto optical system, the chromatic aberrations may be corrected in a unit having front lenses through which paraxial marginal ray and paraxial chief ray pass at a relatively large height from the optical axis. More specifically, the chromatic aberrations are corrected using a lens having a positive refractive power made of a low-dispersion optical material having extraordinary partial dispersion (having a large Abbe number), such as fluorite, and a lens having a negative refractive power made of a high-dispersion optical material.
Such telephoto optical systems have been disclosed in, for example, U.S. Pat. No. 4,241,983 (Japanese Patent Publication No. 60-49883), U.S. Pat. No. 4,348,084 (Japanese Patent Publication No. 60-55805), and U.S. Pat. No. 6,115,188 (Japanese Patent Laid-Open No. 11-119092).
The paraxial marginal ray refers to the paraxial ray that comes parallel to the optical axis of an optical system at a height of 1 from the optical axis with the focal length of the entire optical system normalized to 1. In the following description, the object is described as placed to the left of the optical system, and rays coming into the optical system from the object are described as traveling from left to right.
The paraxial chief ray refers to the paraxial ray that comes at −45° with respect to the optical axis and that passes through the intersection of the entrance pupil and the optical axis of the optical system with the focal length of the optical system normalized to 1. In the following description, the incident angle is defined as being positive when it is in the clockwise direction with respect to the optical axis, and as being negative when it is in the anticlockwise direction.
U.S. Pat. Nos. 5,731,907 and 5,638,215 have disclosed achromatic optical systems using liquid materials exhibiting relatively high dispersion and relatively extraordinary partial dispersion.
In the telephoto optical system using fluorite or the like as an optical material, as disclosed in the foregoing three patent documents, by setting the optical overall length to be relatively large, the chromatic aberrations can be easily corrected. In contrast, a reduced optical overall length can cause chromatic aberrations to occur frequently, and makes it difficult to correct the chromatic aberrations.
This is because this type of telephoto optical system uses an optical material such as fluorite, which has low dispersion and extraordinary partial dispersion, so as to simply reduce the chromatic aberration occurring in the front lens unit with a positive refractive power. In order to correct the chromatic aberrations of an optical system accompanying the reduction of the optical overall length in, for example, a lens system using a low dispersion glass having a high Abbe number, such as fluorite glass, it is necessary to vary the refractive powers at the surfaces of the lenses by a large amount. Accordingly, it is difficult to appropriately correct both the chromatic aberrations and other aberrations occurring by increasing the refractive power, such as spherical aberration, coma, and astigmatism.
The liquid material as disclosed in the above-cited patent documents requires a structure in which it is enclosed. This makes it complicated to manufacture optical systems using such a material. In addition, the liquid material is not sufficiently resistant to the environment and its properties, such as refractive index and dispersion, vary with temperature. Furthermore, the liquid material does not form an interface with air, and it is therefore difficult to correct the chromatic aberrations sufficiently.